What’s is soil?
All soil is made up of inorganic mineral particles, organic matter (including living things), air and water.
Inorganic mineral particles
Inorganic mineral particles make up more than half the volume of soil. These particles come from rocks – the parent material that formed the soil. Soil mineral particles are sorted into three groups based on their size – sand, silt and clay. Sand particles are the biggest and clay particles are the smallest. If you rub soil between your fingers, the sand particles make it feel gritty. Silt soils feel smooth and floury. A slick or sticky feel comes from clay.
Organic matter (living and non-living)
Soil is home to an amazing amount of life. Some living things are big so we can see them, but most aren’t. Soil is full of life – billionsof bacteria, fungi and other microorganisms. Scientists think there is more life in one teaspoon of healthy soil than there are people living on Earth!
The other component of organic matter is humus. It comes from dead plants and animals and the waste products of living things. When we add compost to the soil, we are adding humus.
Organic matter makes up a small part of the soil, but it plays a really important role. The living organisms in soil recycle nutrients. Humus stores the nutrients and water for plants. Organic matter makes it easier to work the soil by loosening it and making it less dense – this allows the plant roots to penetrate and grow more easily and also allows water air and nutrients to permeate.
Air and water
Air and water may make up almost half the volume of healthy soil! Air and water are in small spaces called pores between soil particles. Plants and soil animals need the air and water to live and grow.
Soils vary from area to area
All soils have some things in common. They are all made of mineral particles, organic matter, air and water – but soils are also different due to how and where they were formed.
Five factors influence soil formation: parent material, climate, living organisms, topography and time.
The rock from which a soil is formed is called parent material. Sometimes the parent material stays in the same place, while other parent materials are transported by wind, water or by other means. Parent materials also have different properties, for example, the amount of nutrients. That means soils that began as hard rock from volcanic eruptions will be different to those that began as sediments deposited by rivers.
The simple effects of heating and cooling can speed up the weathering or breaking down of rocks into smaller pieces. Warm temperatures and rain encourage plants and animals to grow, adding organic matter to the soil. Rain also washes (transports) rocks and soil off slopes and can dissolve minerals, adding them to the soil.
How soil is formed
Plant roots grow down into the soil. Roots get into cracks and release chemicals that help make nutrients available. Earthworms and other animals tunnel through soil and mix it up. When plants and animals die, they add organic matter to soil. Human intervention also makes changes to soils. Removing vegetation from the top of soil exposes it to erosion – the soil can get blown or washed away. Humans may also add fertiliser to make soil more productive or lime to make it less acidic.
Topography refers to the land. If land is sloped, gravity moves soil particles downwards, deepening the soil in a valley. Topography also influences the climate. New Zealand weather patterns often come in from the west. The air cools as it rises up over the mountains, and the moisture in the air falls as rain. The air is drier by the time it reaches land in the east, so less rain falls.
Different places, different soils
The soils will be different in each landscape due to the influence of the mountains, the climate and the vegetation.
It takes a long time for soil to develop – from 500 to thousands of years for every 1–2 cm. The age of soils differs around the country.
Soils of Britain
There are over 1,800 British soils with a range of different profile types, despite the modest land area and limited range of cool, temperate climates.
Britain has a wide range of rock types and these generate diverse soil parent materials, ranging from porous sands to impermeable clays and from acid to alkaline. This variety increased over the past two million years, when Britain's climate fluctuated between arctic Ice Ages and warm temperate. Surface materials were moved and sorted by ice, wind and water. This resulted in many mixtures and combinations of soil parent materials from different rock types. Climatic swings also affected the topography and many British landscapes were sculpted by ice and water.
Britain has a very varied climate and geology which has led to a wide range of soils being developed. The annual rainfall varies from about 250mm in East Anglia to over 2,000mm on the top of Mount Snowdon in North Wales. Furthermore, there are significantly higher annual temperatures in Southern England than in the north of Scotland and this affects the rate of weathering of rock to form soils.
Plant cover also influences the way soils form. Since the last Ice Age, when soils began to re-form after being scoured away, there have been many changes in vegetation. As the temperatures warmed up after the Ice Age, so different types of vegetation colonised the soils, each making its mark on soil formation. One of the more recent colonisations was that of deciduous woodland which covered much of Britain except the higher mountains. Under this vegetation the brown earth soil, one of the main soil types of Britain, became widespread
Humans have had a major effect on the soils of Britain. Much of the deciduous forest was cut down in the 16th to the 18th centuries and the land ploughed to make way for agriculture. Since this extensive deforestation much of the land has been cultivated. In recent years there has been intensification of agriculture, with more extensive and sophisticated mechanisation, much increased use of fertiliser, and extended use of pesticides and herbicides. Humans now play a large part in soil development.